1. Field of the Invention
This invention relates in general to methods and devices for reducing interference with wireless communications, and in particular, to a method and apparatus for reducing intermodulation interference in mobile cellular telephones.
2. Description of Related Art
Currently, much of the infrastructure for cellular telephone systems in the United States comprises AMPS (advanced mobile phone system) or analog-type cellular telephone systems. Next-generation CDMA (code division multiple access) cellular telephone systems are now being installed, often in parallel with AMPS cellular telephone systems. These CDMA cellular telephone systems rely on low power broadband spread spectrum transmission techniques. In most circumstances, CDMA and AMPS cellular systems can coexist without significant interference. However, recent experience has shown that interference between CDMA and the AMPS systems is possible under some circumstances. For example, when a CDMA mobile unit approaches an AMPS-only site, the AMPS signal overloads the CDMA mobile cellphone receiver radio frequency (RF) front-end circuits due to the relative strength of the AMPS signal and the relative weakness of the spread-spectrum CDMA signal. Intermodulation (IM) components created by the AMPS signal after low noise amplification (LNA) also interfere with the weak CDMA signal.
Efforts to curb this problem have focused on modifying the CDMA mobile units by the conventional method of adding attenuation pads and changing the range of LNA to reduce IM problems. Unfortunately, these changes are costly and difficult to implement because they rely on design changes to the mobile cellphones, which are large in number. Further, even where this technique has been used, it has not substantially improved reception quality.
Thus, there is a need for an inexpensive and effective system and method for eliminating intermodulation interference in cellular telephone systems.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses an apparatus for reducing AMPS-related interference received by a CDMA cellular telephone mobile receiver. The apparatus comprises a first antenna for receiving the CDMA signal, an interference-eliminating signal generator, and a second antenna for transmitting the interference-eliminating signal. The interference-eliminating signal generator comprises a signal processor and adaptive nuller for adaptively reducing the foreign signal and passing forward channel signals, a low noise amplifier and an attenuator. In one embodiment, the interference-eliminating signal generator further comprises a filter to shape the spectral content of the interference-eliminating signal. In another embodiment, the attenuator is adjustable, and can be adjusted according to the distance between the second antenna and the AMPS transmitter, or according to the measured amplitude of the AMPS receiver. The present invention also incorporates antenna adaptive techniques to null the foreign signal. Similarly, the method comprises the steps of receiving the CDMA signal, and processing the signal to pass the forward channel signals and reject the reverse channel signals, amplifying the signal, and transmitting the amplified CDMA signal to the mobile cellular telephone.